the muzzle as will draw away attention from the 
pain caused by the shoeing or the surgical opera- 
tion. Thecommon barnacles are rollers of wood, 
bound together, and made to enclose and com- 
press the muzzle; another kind have handles, 
and operate like pincers; and a third sort are 
held together at the top by a ring enclosing but- 
tons. Much unnecessary pain is often, from 
inconsideration or cruelty, inflicted with these 
instruments; yet, in some instances, even the 
highest degree of it fails to accomplish the ve- 
terinary surgeon’s purpose, so that he is obliged 
to have recourse to the strong means of restraint 
afforded by the side-line or the hobbles. 
BAROMETER. An instrument for determin- 
ing the weight of the air, and the variations of 
its pressure in different circumstances. As every 
change in the weight of that fluid is accompanied 
with corresponding changes of density, and con- 
sequently of its disposition to absorb or deposit 
| moisture, the barometer is also employed to point 
| out the probable changes of weather ; hence it is 
| not unfrequently called a weather-glass. 
| purpose, scarcely less important, to which this 
| instrument has lately been much applied, is the 
Another 
measurement of accessible heights; and the re- 
' sults obtained by means of it approach so near 
to perfect accuracy, when all circumstances are 
properly estimated, that this method of deter- 
| mining the heights of mountains is, in many 
cases, even preferable to the geometrical methods. 
Tt also appears from the observations of Captain 
Flinders, that the barometer may be of the most 
essential service at sea, not only to foretell changes 
of weather, but also to indicate the vicinity of 
land. These important properties entitle this 
instrument to a considerable share of our atten- 
tion. 
Before the discovery of the weight of the air, 
the barometer was entirely unknown; and, in- 
deed, it was the discovery of that fact, which led 
to the invention of the instrument. Evangelista 
Torricelli, a pupil of Galileo, and his successor as 
mathematical teacher at Florence, was the in- 
ventor of the barometer, about the middle of the 
17th century. He conjectured that the same 
cause which raises water 33 or 34 feet high (see 
ATMOSPHERE), a discovery of Galileo, ought to 
raise mercury, which is nearly 14 times heavier, 
only 29 or 30 inches high. He therefore closed 
a tube of glass, several feet long, hermetically at 
one end, then filled it with mercury through the 
orifice at the other end, and inverted it in a 
vessel of mercury. He was not deceived in his 
expectations: the mercury descended from the 
upper part of the tube, and remained in a column 
29 or 30 inches high. The upper part of the 
tube, which, in this experiment, became empty, 
was thence called the Vorricellian vacuum. Fur- 
ther experiments of Torricelli proved the correct- 
ness of his idea that the column of mercury was 
| supported by the pressure of a column of air 
resting upon the column of mercury in the tube, 
BAROMETER. 
and extending to the limits of the atmosphere. 
Whilst Torricelli was still occupied with this 
object, death overtook him, in 1647. The above- 
described preparation, which is the barometer 
itself, is called, after him, the Torricellian tube. 
Pascal adopted his opinions, and performed many 
experiments in confirmation of them. He re- 
quested one of his relations, Perrier, at Clermont, 
in Auvergne, to make a trial of the pressure of 
the air on the mountain Puy-de-Dome. Perrier 
found that the quicksilver in the Torricellian 
tube, upon the summit of this mountain, 5,000 
feet high, stood more than three Parisian inches 
lower than at the foot of the mountain ; and thus 
demonstrated that it was not the horror of a 
vacuum, as had been previously supposed, but | 
the pressure of the column of air (the height and 
the weight of which were less on the mountain), 
that supported the column of mercury in the 
tube. The gradual fall of the mercury, in as- 
cending the mountain, was also observed. It 
could not escape the notice of the first inventors 
of the barometer, that the situation of the mer- 
cury in the Torricellian tube was almost daily 
changing. They concluded that the pressure of 
the atmosphere must be subject to continual 
changes, and that, on this account, this instru- 
ment would be useful for pointing out and mea- 
suring these changes. 
The experiment of Torricelli was so simple, and 
yet so easy to be exhibited under a variety of 
forms, that a great number of barometers were 
soon proposed, either with a view of rendering | 
them more correct, or enlarging the extent of the | 
Before we proceed to give | 
particular descriptions of these mstruments, and | 
barometrical scale. 
of the various attempts which have been made 
to increase their accuracy and sensibility, it may | 
not be improper to make some previous remarks | 
applicable to barometers in general. 
The tubes intended for barometers ought to be 
sealed hermetically at both ends, immediately 
after they are made at the glass-house, and to be 
kept in that state till they are to be fitted-up. 
Without this precaution, they are apt to be sullied 
with dust, moisture, and other impurities, which 
it is almost impossible afterwards to remove, on 
account of the smallness of their diameters. 
When they are opened—which may be done with 
a file—care should be taken not to breathe into 
them, or to wash them with spirit of wine, or 
any other fluid; experience having proved, that 
in tubes so treated, the mercury always stands a 
little below its proper level. This is, no doubt, 
owing to the adhesion of a little of the spirit of 
wine to the sides of the tube, which being after- 
wards converted into vapour, renders the vacuum 
above the mercury imperfect. If any cleaning is | 
necessary, it may be done with a fine linen rag, 
that has previously been well dried. 
The tubes ought to be as perfectly cylindrical 
as possible, though, in some cases, this is not 
absolutely necessary. They should be about 33 
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